Increased precision position sensors

ABSTRACT

An increased precision position sensor includes a first magnetic field sensing device; a second magnetic field sensing device; and a processor, the processor further includes a calibrator; a mathematical model to predict the magnetic field at a given position relative to the object; and an estimator algorithm to compare the measured magnetic field within the predicted magnetic field, thereby calculating the most likely position of the position sensor relative to the magnetic object. A position sensor in accordance with the invention is capable of locating the axis of a cylindrical magnetic object to within ±0.5 mm through 70 mm thick aluminium, and is expected to find application, in the aerospace industry, or other industries where high precision during manufacture is required. The invention may be conveniently embodied in a portable, handheld device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application is the U.S. national phase of international applicationPCT/GB2004/004674, filed in English on 4 Nov. 2004, which designated theU.S. PCT/GB2004/004674 claims priority to GB Application No. 0325989.2filed 7 Nov. 2003 and EP Application No. 03257035.0 filed 7 Nov. 2003.The entire contents of these applications are incorporated herein byreference.

This invention relates to a system for increasing the precision of aposition sensor. In particular, this invention relates to a system forincreasing the precision of a position sensing system incorporatingmagnetic field sensing devices such as Hall Effect devices.

2. Discussion of Prior Art

Position sensors and position sensing systems are often used inmanufacturing applications where it is necessary to drill holes ‘blind’,that is where it is not possible to see the structure through which thehole is to be drilled. One example of such a manufacturing applicationis found in the aerospace industry when assembling a wing skin and awing box where it is essential to determine accurately where to drillattachment holes through the wing skin and into the supporting feet of arib of the wing box. Erroneous drilling of such attachment holes resultsin the incorrect distribution of stresses and strains through theairframe.

Conventionally, pilot holes are drilled from within the wing boxoutwards through the rib foot and the wing skin, a process known as backdrilling. The pilot holes are then used as a guide for the drilling ofattachment holes. However, since space is restricted within the wingbox, the back drilling process is complex and prone to errors.Correction of these errors is a further time consuming task, andrequires further drilling that may deleteriously effect the robustnessof the resulting structure. There is thus a need for a position sensorto locate the correct drilling location for the attachment holes fromwithout the wing box.

A known position sensor comprises an array of Hall Effect devices usedto sense the magnetic field due to a cylindrical magnetic object placedat the desired location of the attachment hole. The Hall Effect devicesare arranged in concentric circles such that the axis of the cylindricalmagnetic object is located when each Hall Effect device in a givencircle senses the same magnetic field. Such a position sensor is ablefrom above the wing skin, to locate the axis of a cylindrical magneticobject placed on a rib foot within the wing box to within a tolerance of±2.5 mm.

WO2004/016380 also describes a method and apparatus for locatingnon-visible objects. In this known method and apparatus the position ofthe object can be sensed by means of a suitable array of Hall effectsensors which can be moved relative to the object in question.

However, a greater degree of precision in locating the magnetic objectis typically required for such position sensors to be useful in theaerospace industry or other industries where high precision duringmanufacture is required.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the precision ofsuch position sensors.

By application of the software of the present invention to knownposition sensors it is another object of the present invention toaddress at least one of the above-identified disadvantages associatedwith the position sensors currently in use.

It is another object of the present invention to allow sensing of theposition of a magnetic object through thick stacks of intermediatematerial (for example, through thick wing skins).

In broad terms, the invention resides in the concept of using amathematical model to predict the magnetic field associated with themagnetic object, and comparing the predicted magnetic field with themagnetic field measured by the array of Hall Effect devices. Thecomparison can be effected using an estimator algorithm and allows thelocation of the magnetic object to be determined to an improvedprecision.

According to a first aspect of the invention, there is provided aposition sensor for sensing the position of an object having anassociated magnetic field comprising:

a first magnetic field sensing device at a first position that outputs afirst signal related to the magnetic field at the first position;

a second magnetic field sensing device at a second position that outputsa second signal related to the magnetic field at the second position;

a processor to derive from the first signal and the second signal themost likely position of the position sensor relative to the object,wherein the processor comprises:

a first calibrator to calibrate the first magnetic field sensing device,thereby deriving a first measured magnetic field;

a second calibrator to calibrate the second magnetic field sensingdevice, thereby deriving a second measured magnetic field;

a mathematical model to predict the magnetic field at a given positionrelative to the object;

an estimator algorithm to compare the magnetic field predicted by themathematical model with the first measured magnetic field and the secondmeasured magnetic field, thereby calculating the most likely position ofthe position sensor relative to the object.

Advantageously, the improved precision of a position sensor according tothe invention dramatically improves the efficiency of construction tasksin which high precision is required. In particular, the position sensoris able to locate the axis of a cylindrical magnetic object to within±0.5 mm when the position sensor is separated from the cylindricalmagnetic object by a 70 mm thick aluminium stack. Conveniently, such ahigh level of precision is sufficient for the position sensor to be usedin aerospace applications. Use of the position sensor avoids timeconsuming and complex procedures such as back drilling and is likely toreduce the occurrence of errors during wing construction. Correction ofsuch errors requires further drilling that increases the number ofpotential weaknesses in the structure. Use of a position sensoraccording to the invention may thus advantageously increase the usefulservice life of the wing structure. Furthermore, the amount of wastemetal produced is reduced, since a large build up of errors duringconstruction will eventually require wing assembly to be scrapped.

Conveniently, in accordance with an exemplary embodiment of theinvention which will be described hereinafter in detail the object canbe a cylindrical magnetic object. Advantageously, cylindrical magneticobjects are easily obtained, and their associated magnetic field iseasily predicted by known mathematical modeling techniques.Conveniently, in accordance with the exemplary embodiment of theinvention, the magnetic field sensing devices may comprise Hall Effectsensing devices. Hall effect sensing devices are preferable because theyare well known in the art of magnetic field measurement, easy to obtainin a complete package form, and cheap. Furthermore, there exists asimple relationship between the output voltage of a Hall effect sensingdevice and the magnetic field at the position of the device, thussimplifying the calibration process.

In the known position sensor described hereinabove, the operator isprovided only with the signals from the magnetic field sensing devices.A position sensor according to the invention advantageously derives themost likely position of the position sensor relative to the object. Thetime taken to locate the correct drill site is thus reduced since theoperator is not required to further interpret the data provided by theposition sensor.

Preferably, the first calibrator comprises a correction model.Optionally, the correction model comprises a gain term and an offsetterm, although any arbitrary correction model may be used.

Advantageously, the use of the calibrator improves the accuracy andreliability of the position sensor, by correcting for discrepanciesbetween the mathematical model and the signals from the Hall Effectdevices. The calibrator preferably calibrates the Hall Effect devicesindividually. Furthermore, the calibrator fully determines theparameters of the mathematical model.

Preferably, the estimator algorithm comprises an extended Kalman filteralgorithm. Advantageously, since the extended Kalman filter algorithm isa recursive algorithm, the processor is able to continually derive themost likely position of the position sensor relative to the object inreal time. Operation of the position sensor is therefore advantageouslysimplified.

Optionally, during operation of the position sensor the object isseparated from the position sensor by a wing skin. Optionally, duringoperation of the calibrator, the first magnetic field sensing device isat a known position relative to the object and is separated from theobject by a wing skin of predetermined thickness.

Preferably, the estimator comprises a software program. Preferably, thecalibrator comprises a software program.

When both the estimator and the calibrator are in the form of softwareprograms, the position sensor can conveniently be embodied as a portablehand held device. Such a portable device can be used easily by a singleoperator to locate drilling sites precisely without the need foradditional machinery or equipment. Moreover, since the most likelyposition of the position sensor relative to the object is continuallyderived in real time, the operator is able to quickly and easilymaneuver the position sensor around the construction area until thecorrect drill site is found. Such a feature is advantageous for largescale construction tasks, and in particular allows the invention to beconveniently used during the assembly of an aircraft wing.

According to a second aspect of the invention, there is provided amethod of sensing the position of an object having an associatedmagnetic field using a position sensor comprising first and secondmagnetic field sensing devices at first and second positions

the method comprising the steps of:

-   -   (a) sensing a first signal related to the magnetic field at the        first position from the first magnetic field sensing device;    -   (b) sensing a second signal related to the magnetic field at the        second position from the second magnetic field sensing device;    -   (c) calibrating the first magnetic field sensing device, thereby        deriving a first measured field from the first signal;    -   (d) calibrating the second magnetic field sensing device,        thereby deriving a second measured magnetic field from the        second signal;    -   (e) determining a predicted magnetic field at a given position        relative to the object using a mathematical model;    -   (f) comparing the predicted magnetic field with the first and        second measured magnetic fields using an estimator algorithm,        thereby calculating the most likely position of the object        relative to the position sensor.

It is to be appreciated that the invention also resides in a computerprogram comprising program code means for performing the steps describedherein above when the program is run on a computer and/or otherprocessing means associated with the position sensor. Furthermore theinvention also resides in a computer program product comprising programcode means stored on a computer readable medium for performing the stepsdescribed herein above when the program is on a computer and/or otherprocessing means associated with the position sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying drawings in which

FIG. 1 illustrates a Hall Effect sensor device as used to sense theposition of a cylindrical magnetic object.

FIG. 2 illustrates the geometry of a model used to calculate themagnetic field due to the cylindrical magnetic object of FIG. 1.

FIG. 3 illustrates the Hall Effect sensor device of FIG. 1 in plan view.

DETAILED DISCUSSION OF EMBODIMENTS

FIG. 1 shows a Hall Effect sensor device 1 positioned above a wing skin2. The wing skin 2 is to be fastened to supporting structures includinga rib 3. A bolt may be used to attach the wing skin 2 to a rib foot 4.The object of the invention is to locate precisely the desired positionof the bolt such that a bolt hole may be drilled. A cylindrical magneticobject 5 is placed on the rib foot 4 such that the central axis 5 a ofthe cylindrical magnetic object 5 coincides with the central axis of thedesired bolt hole.

FIG. 2 illustrates the geometry of a model used to predict the magneticfield B due to the cylindrical magnetic object 5. The magnetic field Bis radially symmetric about the central axis 5 a of the cylindricalmagnetic object 5. The magnetic field B at position r and can be modeledusing the equation

$\begin{matrix}{B \propto {\frac{M_{o}}{4\pi}{\int{\frac{{M\left( r^{\prime} \right)} \times \left( {r - r^{\prime}} \right)}{\left| {r - r^{\prime}} \right|^{3}}{\mathbb{d}V^{\prime}}}}}} & (1)\end{matrix}$

where M_(o) is a constant, M(r′) is the magnetic dipole moment of apoint dipole at position r and V′ is the volume of the cylindricalmagnetic object 5. The principle of the formula (1) is to treat thecylindrical magnetic object 5 as a volume of point magnetic dipoles. Thefield at r is then the sum of the contributions from each dipole in thevolume V′.

FIG. 3 shows the Hall Effect sensor device 1 in plan view. The HallEffect sensor device 1 comprises sixteen Hall Effect sensors 6(a–d) to9(a–d) that are arranged in concentric circles 12–15 as indicated by thedashed lines. A target 10 indicated by a cross is located at the centerof circles 12–15. The Hall Effect sensors 6(a–d) to 9(a–d) are mountedon four sensor blocks 6, 7, 8 and 9. Blocks 6, 7, 8 and 9 are arrangedso as to form a cross-shape. The center of the cross-shape coincideswith the target 10 at the center of circles 12–15. The sensor blocks 6,7, 8 and 9 are embedded in a potting material 11. There is a hole 16 inthe potting material 11 at the position of the target 10. The hole 16 isfabricated so as to be suitable for a guide drill hole to be made in thewing skin 2 without moving the sensor device 1 when the target 10 ispositioned directly above the central axis of the cylindrical magneticobject 5.

The Hall Effect sensors 6(a–d) to 9(a–d) each output a voltage that islinearly related to the component of magnetic field perpendicular to theplane of the cross-shape formed by the sensor blocks 6, 7, 8 and 9. Thiscomponent of magnetic field can be correlated with the sensor outputvoltages d usingp=Xd+Z  (2)where p is the theoretical field predicted by equation (1), and X and Zare constant for a given magnetic object. Thus for each Hall sensor6(a–d) to 9(a–d) there is a gain X and offset +Z.

An extended Kalman filter algorithm is used to calculate the most likelyposition of the target 10 relative to the central axis of thecylindrical magnetic object 5. The extended Kalman filter is awell-known nearly optimal stochastic recursive estimator applicable tonon-linear systems. Any fitting algorithm suitable for non-linearsystems could also be used. In this embodiment of the invention, theextended Kalman filter algorithm may be operated in both a calibrationmode and a positioning mode. In calibration mode, the sensor is placedwith the target 10 on the axis 5 a of the cylindrical magnetic object 5with a known skin thickness between the center of the magnetic object 5and the target 10. The extended Kalman filter algorithm is then used tocalculate the calibration parameters for each Hall sensor whilst thevariable (x, y, t) that describes the position of the target 10 is heldfixed. The calibration mode also determines unknown constants inequation (1). In the positioning mode, the calibration parameters arefixed whilst the position (x, y, t) is calculated using the Kalmanfilter algorithm. The Hall Effect sensor device 1 may then be moved to anew position and the calculation repeated. These steps are repeateduntil the target is located on the axis 5 a of the cylindrical magneticobject 5.

In an alternative embodiment of the invention, a separate calibrationalgorithm is used. The output voltages from the Hall effect sensors aremeasured with the sensor at a number of known positions relative to thecylindrical magnetic object 5, and with a number of known skinthicknesses between the cylindrical magnetic object 5 and the sensor.The calibration program then calculates optimal values for thecalibration parameters and unknown constants in equation (1) using allthe data thus collected. The extended Kalman filter algorithm is thenused as described above to calculate the position of the sensor relativeto the cylindrical magnetic object.

The invention allows the axis of the cylindrical magnetic object to belocated to a precision of ±0.5 mm when the wing skin 2 is 70 mm thickaluminium. This may be compared to a precision of ±2.5 mm obtained usinga previous Hall Effect position sensing device in which the central axisof a cylindrical magnetic object was located using only the conditionthat the output voltage of the Hall sensors in a given circle 12–15 beequal. A precision of ±2.5 mm is not sufficient for aerospaceapplications. The precision of the invention is sufficient for aerospaceapplications.

Other possible embodiments of the invention will be obvious to theskilled reader. It is not necessary to use a cylindrical magneticobject. Equation (1) is equally applicable to a magnetic object of anyshape. Comparison of the prediction of equation (1) with the measuredsignals from the magnetic field sensing devices can then be performed asdescribed above to effect position sensing. For example, a bar magnetmay be used to sense position in one direction only, and the inventionmay be simply modified to sense rotary position precisely. Any patternof magnetic field sensing devices may also be used. In order for theoperator to locate the axis of a cylindrical magnetic object, a minimumof two magnetic field sensing devices are required. In preferredembodiments of the invention, however, more than two magnetic fieldsensing devices are present. The exact number of magnetic field sensingdevices used may vary. It will also be clear to the skilled reader thatthe invention is applicable in many industrial fields other than theaerospace industry.

It is thus to be appreciated that the present invention can be used in avariety of applications where it is required to provide position sensingthrough an intermediate (non-ferrous) material, for example aircraftwing skins. The skilled person in the relevant art would howeverrecognise that it is possible to conceive of a number of applicationsother than that for which the inventive system was originally developed,which will involve the blind positioning and assembly of components.This could include other applications in the aerospace, automotive andsimilar manufacturing industries. The invention could also be amenableto deployment in robotic systems, if desired.

1. A method of determining a drilling location on a wing skin, such thatan attachment hole can be drilled through the wing skin and a supportingstructure, the method comprising the steps of: (a) placing an objecthaving an associated magnetic field on the supporting structure at thedrilling location; (b) locating a position sensor on the wing skin, theposition sensor comprising first and second magnetic field sensingdevices, said first magnetic field sensing device located at a firstposition and the second magnetic field sensing device located at asecond position, said second position different from said firstposition; (c) calibrating the first magnetic field sensing device,thereby deriving a first calibration; (d) calibrating the secondmagnetic field sensing device, thereby deriving a second calibration;(e) predicting the associated magnetic field using a mathematical modelto obtain a predicted magnetic field; (f) sensing a first signal relatedto the magnetic field at the first position from the first magneticfield sensing device, and using the first calibration to derive a firstmeasured magnetic field from the first signal; (g) sensing a secondsignal related to the magnetic field at the second position from thesecond magnetic field sensing device, and using the second calibrationto derive a second measured magnetic field from the second signal; (h)comparing the predicted magnetic field with the first and secondmeasured magnetic fields using an estimator algorithm, therebycalculating a most likely position of the object relative to theposition sensor; (i) maneuvering the position sensor on the wing skintowards the calculated most likely position; (j) repeating steps (f) to(i) above, until the drilling location is determined; and (k) drillingsaid attachment hole at said drilling location.
 2. A method as claimedin claim 1 wherein the step of calibrating the first magnetic fieldsensing device comprises using a correction model.
 3. A method asclaimed in claim 1 wherein the correction model comprises a gain termand an offset term.
 4. A method as claimed in claim 1 wherein theestimator algorithm comprises an extended Kalman filter algorithm.
 5. Amethod as claimed in claim 1 further comprising the step of continuallyderiving the most likely position of the position sensor relative to theobject in real time.
 6. The method according to claim 1, wherein thestep of calibrating the first magnetic field sensing device comprisesthe step of placing the position sensor at a known position relative tothe object, in said known position the position sensor is separated fromthe object by a wing skin of predetermined thickness.
 7. The methodaccording to claim 1, wherein the object comprises a cylindricalmagnetic object.
 8. The method according to claim 1, wherein themagnetic field sensing devices comprise Hall effect sensing devices. 9.A portable device for performing the method of claim
 1. 10. A computerprogram product comprising a readable storage medium containing computerreadable instructions for controlling a computer to perform steps (c)through (h) of the method of claim 1.